An electrostatic air filter for use in removing particles from air is known from U.S. Pat. No. 5,549,735. The known electrostatic air filter comprises a filter medium, an electrode pair including an insulated electrode and a non-insulated electrode positioned on opposite sides of the filter medium. Electrostatic augmentation of the filtration efficiency is realized by applying a potential difference between the electrodes positioned on opposite sides of the filter medium. The insulated electrode is prepared by a method that is known in the art, such as dipping or spraying a wire or a stamped metal strand, extruding or injection moulding an insulator simultaneously with a wire, and piecing together injection-moulded insulator halves around a wire. The purpose of the insulated electrode is to prevent establishment of a conductive path between the insulated electrode and the noninsulated electrode through the filter medium, such as to prevent electric arcing between the electrodes when a high electric field is established within the filter medium.
The inventors of the present invention have realized that the known electrostatic air filter can exhibit a rapid decrease in filtration efficiency during operation.